In wide-field fluorescent microscopy, halogen or arc lamps are generally used as sources of white light in combination with spectral filters in order to bring electromagnetic radiation of suitable wavelengths for observation or measurement onto an object to be examined or onto a specimen to be examined. However, such sources of white light have a brief service life, so that the light source must frequently be changed. These light sources are also characterized by the development of a good deal of heat, which can have unfavorable effects on the observations and measurements. Furthermore, the spectral portions in the light sources that are not used must to a great extent be suppressed in some manner. An additional disadvantage of these light sources is that it is not possible to turn them on and off rapidly, because the light sources tend to have an afterglow.
Light sources with a light output of greater than 100 mW, which is adequate for wide-field fluorescent microscopy, are known and available. These are primarily color LEDs with a spectral half-width value of approximately 20 to 50 nm. However, there are also white light LEDs that have spectral maximums in the blue and green range of the spectrum. The outputs of color LEDs are comparable to the light output that a halogen or arc lamp has after spectral filtering to approximately 20 to 60 nm bandwidth of the excitation light.
For visual illumination purposes in microscopy, it is the state-of-the-art to overlay three or more LEDs on different wavelengths (RGB light sources) in order to obtain white light, e.g. for projection purposes.
DE 100 17 823 A1 describes a microscopic illumination apparatus with a light source embodied as a light diode arrangement. This diode arrangement can comprise white light diodes or even infrared light diodes and can be embodied such that different types of illumination, such as incident illumination, transmitted light illumination, or combined illumination of the objects to be examined can be realized. It is also possible to attain “oblique” illumination of the object.
The disadvantage of this illumination device is that there is no provision for turning on and placing different LEDs in the illumination ray path of the microscope.